This term has a number of meanings (e.g., brain storming, heuristics), but here it refers to the scientific method. This means having procedures for the systematic pursuit of knowledge involving the recognition and formulation of a problem, the collection of data through observation and experiment, and the formulation and testing of hypotheses. The term is …
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Metazoan
Refers to all vertebrates and invertebrates, but excludes the Protozoa (i.e., unicellular or acellular microorganisms) and Porifora (i.e., sponges). More generally, it refers to the sub-kingdom of multicelluar animals having a body made of cells organised into tissues and coordinated by a nervous system. See Apoptosis (or cell death), Cell recognition molecules, Embryogenesis, Eukaryote cell …
Metencephalon
Housing the cerebellum and pons and many ascending and descending tracts of the reticular formation, it is the rostral (anterior) part of the rhombencephalon (or hindbrain), and sits above the medulla oblongata. It also contains part of the fourth ventricle as well as parts of the fifth (trigeminal), sixth (abducens), seventh (facial) and eighth (vestibulocochlear) …
Metaphor
A figure of speech in which an expression is used to refer to something that it does not literally denote as a means of suggesting a similarity. Metaphors, like analogies, play important roles in the development of scientific theories and in applying theory to a set of metaphorical re-description. It has been argued that when …
Metamorphosis (or indirect development)
The hormonally-driven transition from a larval stage to an adult in which a significant proportion of the organism’s structure changes such that the larval and adult stages are often not recognizable as the same individual. Thus, it is a process of qualitative and radical change in both structure and function from the embryo to the …
Metaphase
Discounting the interphase of meiosis, the second phase of cell division (following prophase) during which the membrane around the nucleus breaks down and the centromeres attach the chromosomes to the equator of the spindle. In the first metaphase of meiosis, pairs of chromosomes are attached, while in mitosis and the second phase of mitosis individual …
Metabolism
The sum of chemical and physical processes occurring within a living organism, but sometimes only used to refer to enzymic reactions. Consists of an alternation of anabolism (the build up and synthesis of complex molecules from simpler ones and the taking up and storing of energy) and catabolism (breaking them down with the release of …
Mesoderm
The middle primary germ layer that originates during gastrulation [the other two being entoderm (inner layer) and the ectoderm (outer layer)], and which gives rise to a large number of structures: muscles, elements of the skeleton (bone and cartilage), the urogenital system, kidneys, gonads and the adrenal cortex. It also differentiates into bone marrow, connective tissues …
Meta-analysis
Systematic and quantitative examination of the results from different studies, pooling them so that robust conclusions can be reached and the nature of agreements or disagreements can be clarified and understood. A important technique in meta-analysis is a funnel plot: a test for publication bias in meta-analyses in which the number of participants in each study …
Mesenchyme
A meshwork of mesoderm-derived undifferentiated loose connective tissue in the embryo, and thus sometimes referred to mesenchymal connective tissue. Derived mainly mesoderm, some of it has its origins in other germ layers, as well as from ectoderm of the neural crest. It consists of a gelatinous ground substance that is made of proteins and water. …